Piggybacking simulcast id and ALR experiment id into video content type extension.

webrtc/modules/pacing/alr_detector.h

Index: webrtc/modules/pacing/alr_detector.h
diff --git a/webrtc/modules/pacing/alr_detector.h b/webrtc/modules/pacing/alr_detector.h
index 7e630970e6345f31fdb399bf322525c071eff26e..2f9c803fecdc17bec129a64489d32e9efa727327 100644
--- a/webrtc/modules/pacing/alr_detector.h
+++ b/webrtc/modules/pacing/alr_detector.h
@@ -47,6 +47,9 @@ class AlrDetector {
     int alr_bandwidth_usage_percent = kDefaultAlrBandwidthUsagePercent;
     int alr_start_budget_level_percent = kDefaultAlrStartBudgetLevelPercent;
     int alr_stop_budget_level_percent = kDefaultAlrStopBudgetLevelPercent;
+    // Will be sent to the receive side for stats slicing.
+    // Can be 0..6, because 3 bits are used and one value is reserved for N/A.
+    int group_id = 0;
   };
   static rtc::Optional<AlrExperimentSettings> ParseAlrSettingsFromFieldTrial();
 
@@ -59,6 +62,7 @@ class AlrDetector {
   static constexpr int kDefaultAlrStartBudgetLevelPercent = 80;
   static constexpr int kDefaultAlrStopBudgetLevelPercent = 50;
   static const char* kScreenshareProbingBweExperimentName;
+  static const char* kScreenshareProbingBweExperimentNameOnReceiveSide;
 
   void UpdateBudgetWithElapsedTime(int64_t delta_time_ms);
   void UpdateBudgetWithBytesSent(size_t bytes_sent);